Origin

Middle English (especially representing the title given to the head of the Roman Empire): from Old Frenchemperere, from Latinimperator 'military commander', from imperare 'to command', from in- 'toward' + parare 'prepare, contrive'.

The root of emperor is the Latin word imperare ‘to command’, which is also the ultimate source of empire (Middle English), imperative (mid 16th century), imperial (Late Middle English), and imperious (late 16th century). Latin imperator meant ‘military commander’, which was given as a title to Julius Caesar and to Augustus, the first Roman emperor, and was adopted by subsequent rulers of the empire. In English, emperor first referred to these Roman rulers, and then to the head of the Holy Roman Empire. See also evil

[After the title of the story Kejserens nye klæder (1837) by the Danish writer Hans Christian Andersen (first translated into English as The Emperor's New Clothes in 1846), in which an emperor is tricked into thinking he is wearing beautiful new clothes, which all his courtiers pretend to admire, until a boy points out that he is in fact naked]